A nonvolatile semiconductor memory device (hereinafter, simply referred to as a memory device) which is capable of writing and erasing data repeatedly, such as an EEPROM or a flash memory, is convenient and has high resistance to physical impact. Therefore, it has been mainly used for a portable memory medium such as a USB flash drive or a memory card, an RF tag which is a medium for radio frequency identification (RFID) in which data is read wirelessly, or the like, and has been widely available on the market. In the memory device, a transistor which functions as a memory element is included in each memory cell. Further, the transistor has an electrode called a floating gate, between a gate electrode and a semiconductor film serving as an active layer. The accumulation of charge in the floating gate enables storage of data.
In Patent Documents 1 and 2, a thin film transistor including a floating gate which is formed over a glass substrate is described.